Process for quenching films



June 28, 1966 w, H. BRwN, JR 3,258,515

PROCESS FOR QUENCHING FILMS Filed May 7, 1965 United States Patent 3,258,515 PROCESS FOR QUENCHING FILMS William H. Brown, Jr., Drexel Hill, Pa., assignor, by mesne assignments, to FMC Corporation, San. Jose, Calif., a corporation of Delaware Filed May 7, 1963, Ser. No. 278,551 8 Claims. (Cl. 264-177) The present invention relates to the manufacture of artificial films, and more particularly to an improved process for quenching shaped streams of molten thermoplastic material to provide films having more desirable properties.

Generally, in the manufacture of artificial films, a molten thermoplastic material is extruded as a shaped stream and is rapidly quenched, as for example by being immersed within a bath of cooling liquid. In another procedure, the shaped stream of molten thermoplastic material is passed downwardly into a generally con-fined pool of liquid which is maintained in the nip formed by a pair of rolls which are rotated at equal peripheral speeds and in the direction of travel of the shaped stream. These contra-rotating rolls are partially immersed Within a bath of cooling liquid so that each carries a thin film of liquid into the nip as it is rotated. Excess liquid is discharged at a controlled rate from opposite ends of the nip so that the pool of liquid is maintained substantially constant.

In both of these conventional procedures the established flow patterns of the cooling liquids fail to provide for uniform quenching of the shaped stream of molten material. More particularly, the passage of the shaped stream of molten material into and through a cooling bath does not set up an effective circulation of the cooling liquid so as to insure uniform quenching of such stream along its entire length. On the other hand, when using a generally confined pool of liquid, such liquid must flow laterally toward the opposite longitudinal edges of the shaped stream of molten material before being discharged and thus the stream is non-uniformly quenched in a transverse direction.

Further, with both of these known procedures, fluctuations, such as ripples, along the surface of the cooling liquid will give rise to thickness or gauge variations and defects along the surfaces of the resulting film. With an open bath such surface fluctuations may be caused by the movement of the shaped stream of molten material itself, while in the last described procedure the lateral flow of the cooling liquid will tend to impart turbulence along the surface of the confined pool. Although such thickness variations and surface defects may not be visible to the naked eye before the film is subjected to furthertreatments, such as stretching the final film produced will generally be found to be of uneven quality. Accordingly, a

primary object of this invention is to provide a generally new or improved and more satisfactory process for quenching shaped streams of molten thermoplastic material.

Another object is the provision of an improved process for more uniformly quenching a shaped stream of molten thermoplastic material with a cooling liquid.

A further object is the provision of an improved process for quenching a shaped stream of molten thermoplastic material vvithin a bath of liquid without imparting surface irregularities or variations in thickness along such stream.

Still further objects will appear from the following description. I

Broadly, the process of the present invention involves passing a freshly extruded shaped stream of molten thermoplastic material downwardly in-between a pair of contra-rotating rolls which are at least partially immersed within a bath of liquid maintained at a temperature below the melting point of the thermoplastic material. The rolls are spaced from each other a distance at least equal to,

Patented June 28, 1968 and preferably slightly more than the thickest portion of the shaped stream of molten thermoplastic material and are together rotated at substantially equal peripheral speeds in the direction of travel of the shaped stream as it moves in-between such rolls.

With the arrangement described, a layer of cooling liquid is carried by the respective contra-rotating rolls toward and against the opposite sides of the shaped stream of molten thermoplastic material and travel with such stream as it passes in-between the rolls. These concurrent flows of cooling liquid assure rapid and uniform quenching of the shaped stream of molten thermoplastic material along its entire length and, additionally, minimize turbulence within as well as surface fluctuations along that portion of the cooling liquid through which the shaped stream first passes. As a result, the film which is produced is generally free of surface irregularities and/or unintentional variations in thickness or gauge.

For satisfactory practice of the process of the present invention, the pair of contra-rotating rolls must be immersed within the cooling liquid bath to a depth equal to between one-half and one times the roll diameter. Preferably, less than one-half of the respective rolls should be exposed, and in no case should the rolls actually be submerged below the bath surface. Further, the peripheral speed of such rolls should be equal to or less than the rate of travel of the shaped stream of molten thermoplastic material so that the concurrent fiows of cooling liquid which are established exert no apparent stretching influence upon such stream during the quenching thereof. In general, the speed of the rolls is reduced as the gap between such rolls is increased.

When quenching a shaped stream of molten thermoplastic material which is of substantially uniform thickness across its entire width, the rollers are spaced from each other a distance slightly greater than the thickness of such stream. On the other hand, when a stream of irregular cross section is being quenched, as for example in the production of beaded edge films, the gap between the rolls should be at least equal to the thickest portion of such stream. When the gap between the rolls is larger than necessary, less efiicient cooling of the shaped stream of molten thermoplastic material results and, in extreme cases, stagnant layers of cooling liquid may remain along the path which is assumed by the shaped stream.

The cooling liquid employed in the process of the invention may be varied as desired to suit the particular thermoplastic material which is being extruded and/ or to satisfy other operating conditions. When applicable, a cooling water bath is preferred in view of the obvious economies and its efiicient quenching action. Further, the process of the present invention is adapted for use with a variety of thermoplastic film-forming materials, as for example ethylene and propylene polymers. For the sake of simplicity and ease of explanation, the process of the present invention is hereafter described in detail as employed in quenching a suitably shaped stream of molten propylene polymer within a bath of cool water to provide a film having a web of substantially uniform cross-section and enlarged or beaded longitudinal edges.

The invention will be more clearly understood by referring to the following detailed description and the accompanying drawing in which:

FIGURE 1 is a longitudinal section taken through an apparatus which is adapted for use with the processes of the present invention; and

FIGURE 2 is a section taken substantially along the line 11-11 of FIGURE 1.

Referring now to the drawing, the illustrated apparatus includes a nozzle 11 for extruding molten polypropylene, a tank 13 for containing a bath 15 of cool water, a pair of cooperating fiow controlling rolls 17 and 19 and a plurality of idler or guide rolls 21, 23 and 25. The nozzle 11 is of the type customarily employed in the manufacture of beaded edge films and may be of a construction as described in U.S. Patent 3,072,962. The water bath 15 is maintained at a temperature below the melting point of the molten polypropylene, preferably by refrigerating and then circulating the same through the tank 13 by means of inlet and outlet conduits 27 and 29, respectively.

The rolls 17 and 19 are of conventional steel construction and, as shown, are immersed within the bath 15 to such an extent as to expose only a relatively small portion of the same above the bath surface 31. Suitable means, not shown, are provided for rotating the rolls 17 and 19 together at the same peripheral speed and in the directions as indicated by the respective arrows 33 and 35. Preferably, the speed of the rolls 17 and 19 is substantially equal to the rate at which the molten polypropylene is extruded from the nozzle 11 to avoid stretching of the extruded stream.

In the practice of the process of the present invention, molten polypropylene is extruded downwardly from the nozzle 11 as a continuous shaped stream 37 which has a central portion 39 of substantially uniform cross-section and enlarged longitudinal edge portions 41. After traveling only a short distance through the surrounding atmosphere, the stream 37 passes into the bath 15 and inbetween the rolls 17 and 19.

The rolls 17 and 19 are spaced apart a distance at least equal to the thickness of the streamedge portions 41 so as to confine a relatively small surface area of the bath 15 without applying any pressure to the stream itself. As the rolls 17 and 19 are rotated in the directions of the arrows 33 and 35, each carries a layer of cool water on its surface which together provide for rapid and uniform quenching of the shaped stream 37 along its entire length. Additionally, the movement of the rolls 17 and 19 causes portions of the bath 15 to flow substantially concurrently with the opposite sides of the shaped stream 37 as it enters and travels between such rolls. As a result, turbulence within and along the bath surface is-minimized so that no distortion of the stream 37 occurs during the quenching thereof.

After the surface of the stream 37 is sufficiently quenched to provide a beaded-edge film, as indicated generally at 43, it is passed about the idler rolls 21 and 23 which maintain the same submerged to insure complete cooling. The film 43 is then withdrawn from the bath 15 and led by the roll 25 to a suitable collection means, not shown. As a result of the quenching process employed, the film 43 exhibits no surface irregularities and/ or unintentional variations in thickness or gauge and is therefore well suitable for further processing, as for example stretching to provide for orientation of the polymer molecules.

It is to be understood that changes and variations may be made without departing from the spirit and scope of the invention as defined in the appended claims.

I claim:

1. A process for quenching a shaped stream of molten thermoplastic material including the steps of passing the shaped stream downwardly in-between a pair of contrarotating rolls immersed within a bath of cooling liquid, said liquid being maintained at a temperature which is below the melting point of the thermoplastic material so as to rapidly quench the shaped stream to a solid'state, said rolls being spaced from each other a distance slightly greater than the thickest portion of the shaped stream and being rotated at substantially equal peripheral speeds in the direction of travel of the shaped stream whereby the cooling liquid adjacent to opposite sides of the shaped stream of thermoplastic material flows with the stream as it enters and travels between the rolls.

2. A process as defined in claim 1 wherein the rolls are immersed in the liquid bath to a depth not greater than their diameter.

3, A process as defined in claim 1 wherein the rolls are immersed in the liquid bath to a depth equal to between one-half and one times their diameter.

4. A process as defined in claim 1 wherein the rolls are rotated at a speed not greater than the rate of travel of the shaped stream of thermoplastic material.

5. A process for making a beaded edge film including the steps of shaping a stream of molten thermoplastic material to provide the same with a central portion of substantially uniform thickness across its width and longitudinal edge portions which are of greater thickness than the central portion thereof, passing the shaped stream downwardly and in-between a pair of contra-rotating rolls immersed within a bath of cooling liquid, said bath being maintained at a temperature which is below the melting point of the thermoplastic material so as to rapidly quench the shaped stream into a solid beaded-edge film, said rolls being spaced from each other a distance at least equal to the thickness of the edge portions of the shaped stream and being rotated at substantially equal peripheral speeds in the direction of travel of the shaped stream whereby the cooling liquid is caused to travel concurrently with and along opposite sides of the central portion of the shaped stream as the stream enters and travels between the rolls.

6. A process as defined in claim 5 wherein the rolls are immersed within the liquid bath a distance equal to between one-half and one times their diameter.

7. A process as defined in claim 6 wherein the rolls are spaced from each other a distance equal to the thickness of the edge portions of the shaped stream of thermoplastic material and are rotated at peripheral speeds substantially equal to the rate of travel of the shaped stream as it enters therebetween.

8. A process as defined in claim 6 wherein the rolls are spaced from each other a distance slightly greater than the thickness of the edge portions of the shaped stream of thermoplastic material and are rotated at peripheral speeds slightly less than the rate of travel of the shaped stream as it enters therebetween.

References Cited by the Examiner UNITED STATES PATENTS 2,585,156 2/1952 Montross 264146 2,728,951 1/1956 OHanlon et al 264l78 X ALEXANDER H. BRODMERKEL, Primary Examiner. F. S. WHISENHUNT, Assistant Examiner. 

1. A PROCESS FOR QUENCHING A SHAPED STREAM OF MOLTEN THERMOPLASTIC MATERIAL INCLUDING THE STEPS OF PASSING THE SHAPED STREAM DOWNWARDLY IN-BETWEEN A PAIR OF CONTRAOROTATING ROLLS IMMERSED WITHIN A BATH OF COOLING LIQUID, SAID LIQUID BEING MAINTAINED AT A TEMPERATURE WHICH IS BELOW THE MELTING POINT OF THE THERMOPLASTIC MATERIAL SO AS TO RAPIDLY QUENCH THE SHAPED STREAM TO A SOLID STATE, SAID ROLLS BEING SPACED FROM EACH OTHER A DISTANCE SLIGHTLY GREATER THAN THE THICKEST PORTION OF THE SHAPED STREAM AND BEING ROTATED AT SUBSTANTIALLY EQUAL PERIPHERAL SPEEDS IN THE DIRECTION OF TRAVEL OF THE SHAPED STREAM WHEREBY THE COOLING LIQUID ADJACENT TO OPPOSITE SIDES OF THE SHAPED 